Asymmetric composite separator, preparation method therefor, and secondary battery

Abstract

An asymmetric composite separator, a preparation method therefor, and a secondary battery. The asymmetric composite separator comprises a base film, a first coating disposed on one side of the base film, and a second coating disposed on the other side of the base film; the first coating comprises a first spherical material grafted with a hydrophilic group, and the second coating comprises a combination of ceramic particles and a second spherical material. The symmetrical composite separator can satisfy bonding requirements of a separator with regard to a negative electrode and a positive electrode in a thermal compounding process, as well as avoid any problems of rollers sticking during a preparation process. In addition, a thermal composite process lithium battery, prepared based on the asymmetric separator, when fully charged and disassembled, has uniform negative electrode interface and does not have problems of material falling, and the cycle performance of the battery is also improved.

Description

Asymmetric composite diaphragm, preparation method thereof, and secondary battery

[0001] This article claims priority to the Chinese patent application filed with the China Patent Office on September 27, 2023, with application number 202311263722.1, and the entire contents of the above application are incorporated herein by reference. Technical Field

[0002] This article belongs to the technical field of diaphragm materials, and specifically relates to an asymmetric composite diaphragm, a preparation method thereof, and a secondary battery. Background Art

[0003] The assembly process of lithium batteries is mainly divided into a winding process and a stacking process. In order to further improve the production efficiency of batteries, the prior art discloses a hot composite stacking process developed on the basis of the traditional stacking process.

[0004] The specific process of the thermal composite lamination process is as follows: first, the separator and negative electrode are bonded together using hot rollers, then cut to form a sandwich structure with two layers of separator covering the negative electrode; second, this sandwich structure is laminated with the positive electrode to form an electrode assembly; finally, the electrode assembly is hot-pressed and bonded to increase its hardness for subsequent circulation. Compared to traditional lamination processes, the thermal composite process eliminates the separate die-cutting step and increases lamination speed by over 50% compared to traditional Z-shaped lamination processes. It has become a development trend in the lithium battery industry.

[0005] At present, the diaphragms used in the thermal composite process are generally prepared by oil-based PVDF coating or water-based PVDF roller coating. During the process, problems such as PVDF adhesive layer sticking to the roller leading to production abnormalities or excessive adhesion between the diaphragm and the electrode leading to negative electrode material falling off are often encountered. In addition, the cost of oil-based PVDF coated diaphragms is twice that of ordinary water-based diaphragms. The above-mentioned high cost and process abnormalities limit the large-scale application of the thermal composite process.

[0006] In addition, in the thermal composite process, hot roller pressing is usually used to bond the diaphragm and the negative electrode, and flat plate hot pressing is used to bond the diaphragm and the positive electrode. Different double-sided hot pressing methods have different requirements for the use of the adhesive coating in the diaphragm. The diaphragm adhesive coating used in the prior art mostly adopts a symmetrical structure. The symmetrical structure of the adhesive layer design can only fully meet the use requirements of one side of the electrode. For example, the following problems exist: If a double-sided coated oily polyvinylidene fluoride adhesive layer is used, the oily polyvinylidene fluoride is prone to roller sticking problems. In order to avoid the above problems, a polyethylene terephthalate film is used to coat the surface of the diaphragm. Covering and hot pressing are performed, but this increases the production cost of the material and the complexity of the equipment; if a double-sided sprayed water-based polyvinylidene fluoride adhesive layer is used, the coverage rate of the sprayed polyvinylidene fluoride is low, and the diaphragm and the negative electrode cannot be well bonded, and the hot pressing pressure needs to be increased to compensate. The increase in hot pressing pressure increases the risk of the diaphragm being punctured by foreign objects and causing short circuits; if a double-sided roller-coated water-based polyvinylidene fluoride adhesive layer is used, since the volume of the negative electrode material continues to change during the charge and discharge cycle of the battery, active materials are likely to fall off between the negative electrode material and the foil, resulting in poor cycle performance of the battery.

[0007] Therefore, in this field, there is an urgent need to develop a composite separator that can avoid the above-mentioned problems existing in the thermal composite process and can also meet the use requirements of the battery.

[0008] Summary of the Invention

[0009] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0010] To address the shortcomings of existing technologies, this paper aims to provide an asymmetric composite separator, a method for preparing it, and a secondary battery. The asymmetric composite separator provided herein can simultaneously meet the bonding requirements of the separator to both the negative and positive electrodes during the thermal composite process, without any sticking to the rollers during the preparation process. Furthermore, thermal composite lithium batteries prepared using this asymmetric separator exhibit a uniform negative electrode interface and no material dropout during full-charge disassembly, significantly improving battery cycle performance.

[0011] To achieve the purpose of this application, this article adopts the following technical solutions:

[0012] In a first aspect, the present invention provides an asymmetric composite membrane, comprising a base membrane, a first coating layer disposed on one side of the base membrane, and a second coating layer disposed on the other side of the base membrane;

[0013] The first coating comprises a first spherical material grafted with hydrophilic groups;

[0014] The second coating includes a combination of ceramic particles and a second spherical material.

[0015] To address the technical problem of existing separators failing to fully meet the requirements of thermal composite processes, this paper designs an asymmetric composite separator. On the one hand, a first spherical material with grafted hydrophilic groups is placed on the positive electrode side to reduce surface energy and the adhesive layer's adhesion to the hot roller. On the other hand, a combination of ceramic particles and a second spherical material is placed on the negative electrode side to achieve adhesion between the separator and the negative electrode, while also improving the separator's heat resistance. Furthermore, the adhesion between the second spherical material and the negative electrode is point contact, with a certain distance between the points. This allows space for stress release when the negative electrode expands during battery cycling, reducing the risk of negative electrode dropout caused by stress concentration during negative electrode volume changes.

[0016] The asymmetric composite diaphragm provided in this article can simultaneously meet the adhesion requirements of the diaphragm to the negative electrode and the positive electrode in the thermal composite process, and there is no roller sticking problem in the preparation process. In addition, the thermal composite process lithium battery prepared based on the asymmetric diaphragm not only has a uniform negative electrode interface and no material falling problem when fully charged and disassembled, but also has greatly improved battery cycle performance.

[0017] In one embodiment, the average particle size of the first spherical material grafted with hydrophilic groups is 100-300 nm, for example, 100 nm, 120 nm, 150 nm, 180 nm, 200 nm, 220 nm, 250 nm, 280 nm, 300 nm, etc.; further, it can be 200 nm.

[0018] In this article, by regulating the average particle size of the first spherical material, the coating height and the pore-blocking effect of the coating material can be controlled. If the particle size is too small, the pores of the spherical material will be blocked, affecting the internal resistance of the battery. Conversely, the coating thickness will be too thick due to the large spherical size, making it difficult to put the battery into the shell.

[0019] In one embodiment, the monomers grafted with hydrophilic groups include fluorine-containing monomers and non-fluorine-containing monomers.

[0020] In one embodiment, the fluorine-containing monomer includes any one or a combination of at least two of polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl chloride, tetrafluoroethylene-ethylene copolymer, polychlorotrifluoroethylene or chlorotrifluoroethylene copolymer.

[0021] In one embodiment, the fluorine-free monomer includes any one of polymethyl methacrylate, polyethyl methacrylate, polymethylethyl methacrylate, polybutyl methacrylate, styrene, styrene propene, phenylpropanol, phenylpropanaldehyde or phenylpropionic acid, or a combination of at least two thereof.

[0022] In one embodiment, the grafting rate of the first spherical material grafted with hydrophilic groups is 2-4%, for example, it can be 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, etc.; it can further be 3%.

[0023] In this article, the grafting rate of the first spherical material is adjusted so that the adhesion of the material meets the requirement of non-stick roller. If the grafting rate is too low, the pole group will become soft, otherwise it will cause the diaphragm coating to stick to the roller.

[0024] In one embodiment, the coating surface density of the first spherical material grafted with hydrophilic groups in the first coating layer is 0.3-0.7 g / m 2 , for example, it can be 0.3g / m 2 , 0.32g / m 2 , 0.35g / m 2 , 0.38g / m 2 , 0.4g / m 2 , 0.42g / m 2 , 0.45g / m 2 , 0.48g / m 2 , 0.5g / m 2 , 0.52g / m 2 , 0.55g / m 2 , 0.58g / m 2 , 0.6g / m 2 , 0.62g / m 2 , 0.65g / m 2 , 0.68g / m 2 , 0.7g / m 2 wait.

[0025] In this article, by regulating the coating surface density of the first spherical material, the asymmetric composite membrane can meet the requirements of adhesion and electrical performance. If the coating surface density is too low, the pole group will become soft, otherwise the coating will clog the pores, seriously affecting the internal resistance of the battery.

[0026] In one embodiment, the coating thickness of the first spherical material grafted with hydrophilic groups in the first coating layer is 0.5-1.5 μm, for example, it can be 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, etc.

[0027] In this article, by regulating the coating thickness of the first spherical material, the asymmetric composite diaphragm is made to meet the adhesion and electrode group thickness design. If the thickness is too small, the coating and the electrode will not be in sufficient contact during hot pressing, affecting the adhesion performance. Otherwise, the diaphragm will be too thick and the electrode group thickness will exceed the standard.

[0028] In one embodiment, the ceramic particles include any one of aluminum oxide, boehmite, magnesium oxide, or lithium aluminum titanium phosphate, or a combination of at least two thereof.

[0029] In one embodiment, the average particle size of the second spherical material is 400-600 nm, for example, 400 nm, 420 nm, 450 nm, 480 nm, 500 nm, 520 nm, 550 nm, 580 nm, 600 nm, etc.; further, it can be 500 nm.

[0030] In this article, by regulating the average particle size of the second spherical material, the asymmetric composite diaphragm meets the bonding design. If the particle size is too small, it will be completely buried in the ceramic coating and will not play a bonding role. Conversely, the particles will be too large and will easily fall off during use, resulting in the bonding performance being unable to meet the use requirements.

[0031] In one embodiment, the second spherical materials are distributed in the second coating layer in the form of islands.

[0032] In one embodiment, the coverage of the second spherical material in the second coating layer is 3-10%, for example, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc.; further optionally, 5%.

[0033] In this article, by regulating the coverage rate of the second spherical material, the asymmetric composite diaphragm meets the bonding design without affecting the electrolyte wetting performance. If the coverage rate is too low, it will not play a bonding role. Conversely, it will reduce the electrolyte wetting space and affect the wetting performance.

[0034] In one embodiment, the second spherical material comprises a fluorine-free material.

[0035] In one embodiment, the fluorine-free material includes any one of styrene acrylic compounds, polymethyl methacrylate, polyethyl methacrylate, polymethylethyl methacrylate, or polybutyl methacrylate, or a combination of at least two thereof.

[0036] In one embodiment, the coating thickness of the ceramic particles in the second coating layer is 1.5-3 μm, for example, it can be 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, 3 μm, etc.; it can further be 3 μm.

[0037] In this article, by adjusting the coating thickness of ceramic particles, the spherical material can form an island structure on the coating surface and is not easy to fall off. If the thickness is too small, the spherical material will be exposed too much in the coating and will be easy to fall off. Conversely, the spherical material will be buried in the coating and cannot form an island structure.

[0038] In one embodiment, the coating thickness of the second spherical material in the second coating layer is 2.5-7.5 μm, for example, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, etc.

[0039] In this article, by adjusting the coating thickness of the second spherical material, the thickness of the asymmetric composite diaphragm meets the design requirements of the battery cell. If the thickness is too small, the electrode group will be too thin, causing shaking after entering the shell. Otherwise, it will make it difficult for the electrode group to enter the shell.

[0040] In a second aspect, this document provides a method for preparing the asymmetric composite membrane according to the first aspect, the method comprising the following steps:

[0041] Mixing the first spherical material grafted with a hydrophilic group, a first binder and water to obtain a first coating slurry, and then coating the first coating slurry on one side of the base film, and drying it to obtain the first coating;

[0042] The second spherical material, ceramic particles, a second binder and water are mixed for a second time to obtain a second coating slurry, and then the second coating slurry is coated on the other side of the base film, and the second coating is obtained after secondary drying.

[0043] In one embodiment, the grafting method of the first spherical material grafted with hydrophilic groups comprises a UV light cross-linking method.

[0044] In one embodiment, based on weight parts, the weight part of the first spherical material grafted with hydrophilic groups is 5-10 parts, the weight part of the first binder is 3-12 parts, and the weight part of water is 78-92 parts.

[0045] Herein, the weight portion of the first spherical material grafted with a hydrophilic group is 5-10 parts, for example, 5 parts, 6 parts, 7 parts, 8 parts, 9 parts, 10 parts, etc.

[0046] Herein, the weight proportion of the first binder is 3-12 parts, for example, 3 parts, 4 parts, 5 parts, 6 parts, 7 parts, 8 parts, 9 parts, 10 parts, 11 parts, 12 parts, etc.

[0047] Herein, the weight percentage of water is 78-92 parts, for example, it can be 78 parts, 82 parts, 85 parts, 88 parts, 90 parts, 92 parts, etc.

[0048] In one embodiment, the coating method is gravure roller coating.

[0049] In one embodiment, the primary drying is performed at a temperature of 80° C. and for 10 minutes.

[0050] In one embodiment, based on weight parts, the total weight parts of the second spherical material and the ceramic particles are 35 parts by weight, the weight parts of the second binder are 6 parts by weight, and the weight parts of water are 59 parts by weight.

[0051] In one embodiment, the mass ratio of the second spherical material to the ceramic particles is (3-12):(88-97), for example, it can be 3:88, 4:88, 6:90, 8:92, 10:95, 12:97, etc.

[0052] In one embodiment, the coating method is gravure roller coating.

[0053] In one embodiment, the secondary drying is carried out at a temperature of 85° C. and for 15 minutes.

[0054] In a third aspect, this document provides a secondary battery, comprising a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator, wherein the separator is the asymmetric composite separator according to the first aspect.

[0055] In this article, the assembly process of the secondary battery adopts a hot composite lamination process, in which the first coating side of the composite diaphragm is set on the positive electrode side, and the hot pressing condition of the hot composite roller on the composite diaphragm is a temperature of 60-100°C, and can be further optionally 90°C.

[0056] In the hot composite lamination process, the second coating layer of the composite diaphragm is arranged on the negative electrode side, wherein the hot pressing condition of the hot composite roller on the diaphragm is a temperature of 60-100°C, and can further be 90°C.

[0057] Compared with the existing technology, this article has the following beneficial effects:

[0058] This paper presents an asymmetric composite separator. This structure employs a first spherical material with grafted hydrophilic groups on the positive electrode side to reduce surface energy and the adhesive layer's adhesion to the hot roller. Furthermore, a combination of ceramic particles and a second spherical material is placed on the negative electrode side, achieving adhesion between the separator and the negative electrode while also improving the separator's heat resistance. Furthermore, the adhesion between the second spherical material and the negative electrode is point contact, with a certain distance between the points. This allows for stress relief during the battery's cycling, minimizing the risk of negative electrode dropout caused by stress concentration during negative electrode volume changes.

[0059] The asymmetric composite diaphragm provided in this article can simultaneously meet the adhesion requirements of the diaphragm to the negative electrode and the positive electrode in the thermal composite process, and there is no roller sticking problem in the preparation process. In addition, the thermal composite process lithium battery prepared based on the asymmetric diaphragm not only has a uniform negative electrode interface and no material falling problem when fully charged and disassembled, but also has greatly improved battery cycle performance.

[0060] Still other aspects will become apparent upon reading and understanding the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] FIG1 is a schematic structural diagram of the asymmetric composite diaphragm provided in Example 1. DETAILED DESCRIPTION

[0062] The technical solution of this invention is further described below by combining the accompanying drawings and specific embodiments. It should be understood by those skilled in the art that the embodiments are only used to help understand this invention and should not be regarded as specific limitations of this invention.

[0063] Example 1

[0064] This embodiment provides an asymmetric composite diaphragm, as shown in Figure 1, the asymmetric composite diaphragm includes a base membrane (the wet-process diaphragm is the base membrane, with a thickness of 9 μm and a porosity of 40%, purchased from Shanghai Enjie Company), a first coating arranged on one side of the base membrane, and a second coating arranged on the other side of the base membrane; the first coating includes a polyvinylidene fluoride spherical material grafted with acrylic acid groups; the second coating includes a combination of boehmite particles and styrene acrylic rubber spherical materials.

[0065] The average particle size of the polyvinylidene fluoride spherical material grafted with acrylic acid groups in the first coating layer is 185 nm, the grafting rate is 3%, and the coating surface density is 0.5 g / m 2 , the coating thickness is 1.2μm; the average particle size of the styrene acrylic rubber spherical material in the second coating is 500nm, the styrene acrylic rubber spherical material is distributed in the second coating in the form of islands, the coverage rate of the styrene acrylic rubber spherical material in the second coating is 5%, the coating thickness of the boehmite particles is 2μm, and the coating thickness of the styrene acrylic rubber spherical material is 5.5μm.

[0066] This embodiment also provides a method for preparing the above-mentioned asymmetric composite diaphragm, which comprises the following steps:

[0067] A polyvinylidene fluoride spherical material (purchased from Sinochem Lantian Company) was grafted with an acrylic acid monomer on its surface by ultraviolet crosslinking to obtain a polyvinylidene fluoride spherical material grafted with acrylic acid groups. 10 parts of the polyvinylidene fluoride spherical material grafted with acrylic acid groups, 5 parts of a polyacrylic acid binder, and 85 parts of deionized water were mixed once by weight to obtain a first coating slurry. The first coating slurry was then coated on one side of the base film by a gravure roller, and dried at 80° C. for 10 minutes to obtain the first coating layer.

[0068] According to the weight ratio, 35 parts of the total weight of styrene acrylic spherical material (purchased from Haodian Technology Co., Ltd.) and boehmite particles (the mass ratio of styrene acrylic spherical material to boehmite particles is 5:95), 6 parts of polyacrylic acid binder and 59 parts of deionized water are mixed for the second time to obtain a second coating slurry, and then the second coating slurry is coated on the other side of the base film with a gravure roller, and the second coating is obtained after drying at 85°C for 10 minutes.

[0069] Example 2

[0070] This embodiment provides an asymmetric composite diaphragm, which includes a base membrane (the wet-process diaphragm is the base membrane, with a thickness of 9 μm and a porosity of 40%, purchased from Shanghai Enjie Company), a first coating arranged on one side of the base membrane, and a second coating arranged on the other side of the base membrane; the first coating includes a polyvinylidene fluoride spherical material grafted with acrylic acid groups; the second coating includes a combination of boehmite particles and styrene acrylic rubber spherical materials.

[0071] The average particle size of the polyvinylidene fluoride spherical material grafted with acrylic acid groups in the first coating layer is 200 nm, the grafting rate is 2%, and the coating surface density is 0.5 g / m 2 , the coating thickness is 0.8μm; the average particle size of the styrene acrylic rubber spherical material in the second coating is 400nm, the styrene acrylic rubber spherical material is distributed in the second coating in the form of islands, the coverage of the styrene acrylic rubber spherical material in the second coating is 4%, the coating thickness of the boehmite particles is 2μm, and the coating thickness of the styrene acrylic rubber spherical material is 3μm.

[0072] This embodiment also provides a method for preparing the above-mentioned asymmetric composite diaphragm, which comprises the following steps:

[0073] A polyvinylidene fluoride spherical material (purchased from Sinochem Lantian Company) was grafted with an acrylic acid monomer on its surface by ultraviolet crosslinking to obtain a polyvinylidene fluoride spherical material grafted with acrylic acid groups. 10 parts of the polyvinylidene fluoride spherical material grafted with acrylic acid groups, 5 parts of a polyacrylic acid binder, and 85 parts of deionized water were mixed once by weight to obtain a first coating slurry. The first coating slurry was then coated on one side of the base film by a gravure roller, and dried at 80° C. for 10 minutes to obtain the first coating layer.

[0074] According to the weight ratio, 35 parts of the total weight of styrene acrylic spherical material (purchased from Haodian Technology Co., Ltd.) and boehmite particles (the mass ratio of styrene acrylic spherical material to boehmite particles is 5:95), 6 parts of polyacrylic acid binder and 59 parts of deionized water are mixed for the second time to obtain a second coating slurry, and then the second coating slurry is coated on the other side of the base film with a gravure roller, and the second coating is obtained after drying at 85°C for 10 minutes.

[0075] Example 3

[0076] This embodiment provides an asymmetric composite diaphragm, which includes a base membrane (the wet-process diaphragm is the base membrane, with a thickness of 9 μm and a porosity of 40%, purchased from Shanghai Enjie Company), a first coating arranged on one side of the base membrane, and a second coating arranged on the other side of the base membrane; the first coating includes a polyvinylidene fluoride spherical material grafted with acrylic acid groups; the second coating includes a combination of boehmite particles and styrene acrylic rubber spherical materials.

[0077] The average particle size of the polyvinylidene fluoride spherical material grafted with acrylic acid groups in the first coating layer is 250 nm, the grafting rate is 4%, and the coating surface density is 0.6 g / m 2 , the coating thickness is 1.2μm; the average particle size of the styrene acrylic rubber spherical material in the second coating is 550nm, the styrene acrylic rubber spherical material is distributed in the second coating in the form of islands, the coverage rate of the styrene acrylic rubber spherical material in the second coating is 8%, the coating thickness of the boehmite particles is 2.5μm, and the coating thickness of the styrene acrylic rubber spherical material is 7μm.

[0078] This embodiment also provides a method for preparing the above-mentioned asymmetric composite diaphragm, which comprises the following steps:

[0079] A polyvinylidene fluoride spherical material (purchased from Sinochem Lantian Company) was grafted with an acrylic acid monomer on its surface by ultraviolet crosslinking to obtain a polyvinylidene fluoride spherical material grafted with acrylic acid groups. 10 parts of the polyvinylidene fluoride spherical material grafted with acrylic acid groups, 5 parts of a polyacrylic acid binder, and 85 parts of deionized water were mixed once by weight to obtain a first coating slurry. The first coating slurry was then coated on one side of the base film by a gravure roller, and dried at 80° C. for 10 minutes to obtain the first coating layer.

[0080] According to the weight ratio, 35 parts of the total weight of styrene acrylic spherical material (purchased from Haodian Technology Co., Ltd.) and boehmite particles (the mass ratio of styrene acrylic spherical material to boehmite particles is 5:95), 6 parts of polyacrylic acid binder and 59 parts of deionized water are mixed for the second time to obtain a second coating slurry, and then the second coating slurry is coated on the other side of the base film with a gravure roller, and the second coating is obtained after drying at 85°C for 10 minutes.

[0081] Example 4

[0082] The difference between this embodiment and embodiment 1 is that the grafting rate of the polyvinylidene fluoride spherical material grafted with acrylic acid groups in the first coating layer is 1%, and the other aspects are the same as those in embodiment 1.

[0083] Example 5

[0084] The difference between this embodiment and embodiment 1 is that the grafting rate of the polyvinylidene fluoride spherical material grafted with acrylic acid groups in the first coating layer is 8%, and the other aspects are the same as those in embodiment 1.

[0085] Example 6

[0086] The difference between this embodiment and embodiment 1 is that the coating surface density of the polyvinylidene fluoride spherical material grafted with acrylic acid groups in the first coating layer is 0.1 g / m 2 , and the rest are the same as in Example 1.

[0087] Example 7

[0088] The difference between this embodiment and embodiment 1 is that the coating surface density of the polyvinylidene fluoride spherical material grafted with acrylic acid groups in the first coating layer is 1 g / m 2 , and the rest are the same as in Example 1.

[0089] Example 8

[0090] The difference between this embodiment and embodiment 1 is that the coverage rate of the styrene acrylic spherical material in the second coating layer is 1%, and the rest is the same as embodiment 1.

[0091] Example 9

[0092] The difference between this embodiment and embodiment 1 is that the coverage rate of the styrene acrylic spherical material in the second coating layer is 15%, and the other aspects are the same as those in embodiment 1.

[0093] Example 10

[0094] The difference between this embodiment and embodiment 1 is that the coating thickness of the boehmite particles in the second coating layer is 1 μm, and the coating thickness of the styrene acrylic spherical material is 10 μm. The other aspects are the same as those in embodiment 1.

[0095] Example 11

[0096] The difference between this embodiment and embodiment 1 is that the coating thickness of the boehmite particles in the second coating layer is 6 μm, and the coating thickness of the styrene acrylic spherical material is 1 μm. The other aspects are the same as those in embodiment 1.

[0097] Comparative Example 1

[0098] This comparative example provides a commercialized glue-coated diaphragm product produced by Shanghai Enjie Company, with a product specification of 9+2+1+1, wherein 9 represents the thickness of the wet-process diaphragm base film, and its porosity is 40%; 2 represents a single-sided coating of 2μm boehmite ceramic material; 1+1 represents a polyvinylidene fluoride coating product with a symmetrical structure coated on the outermost two sides of the diaphragm, wherein the polyvinylidene fluoride powder adopts the Sinochem Blue Sky product number polyvinylidene fluoride, and the coating thickness on both sides is 1μm.

[0099] Comparative Example 2

[0100] The difference between this comparative example and Example 1 is that both the first coating layer and the second coating layer include polyvinylidene fluoride spherical materials grafted with acrylic acid groups, and the rest are the same as Example 1.

[0101] Comparative Example 3

[0102] The difference between this comparative example and Example 1 is that both the first coating layer and the second coating layer include a combination of boehmite particles and styrene acrylic spherical materials, and the rest are the same as Example 1.

[0103] Examples 1 to 11 and Comparative Examples 1 to 3

[0104] The asymmetric composite diaphragms provided in Examples 1 to 11 and Comparative Examples 1 to 3 were assembled into lithium-ion batteries, and the preparation method was as follows:

[0105] Preparation of positive electrode sheets: Lithium iron phosphate is used to prepare a slurry, which is coated on a carbon-coated aluminum foil and treated to produce a positive electrode sheet. The positive electrode accounts for 96.55% of the coating, and the rest is 2.2% polyvinylidene fluoride adhesive, 1% conductive carbon black and 0.25% carbon nanotubes.

[0106] Preparation of negative electrode sheets: Artificial graphite is used to prepare slurry, and the slurry is coated on copper foil to make the treated negative electrode sheet. The negative electrode accounts for 96.1% of the coating, and the rest is 1.1% sodium carboxymethyl cellulose adhesive, 1% conductive carbon black and 1.8% styrene-butadiene latex.

[0107] Electrode assembly fabrication: The asymmetric separator and negative electrode are bonded together using a thermal lamination process using hot rollers and hot pressing at a temperature of 90°C. The resulting unit is cut into individual pieces using a hot cutter, forming a separator-negative electrode-separator sandwich. The sandwich and positive electrode sheets are stacked to form an electrode assembly, which is then hot-pressed at 90°C using a flat-plate hot press to complete the assembly.

[0108] Packaging and liquid injection: The prepared electrode group is packaged in a soft package form and then the electrolyte is injected.

[0109] Pre-charging and formation: pre-charging and forming the battery cells to produce lithium-ion batteries.

[0110] Test conditions

[0111] The asymmetric composite diaphragms provided in Examples 1 to 11 and Comparative Examples 1 to 3 were tested using the following test methods:

[0112] Adhesion test: The membrane was folded in half to test the adhesion. The test conditions were 90°C, 1 MPa surface pressure, and 60 seconds.

[0113] The lithium-ion batteries provided in Application Examples 1 to 11 and Comparative Application Examples 1 to 3 were tested using the following test methods:

[0114] Cycling performance test: At 25°C, the battery was charged at a rate of 0.5C and discharged at a rate of 0.5C for cycling test.

[0115] The test results are shown in Table 1:

[0116] Table 1

[0117] As can be seen from Table 1, the composite separators provided in Examples 1 to 3 herein have good bonding properties and the batteries prepared therefrom have a high capacity retention rate.

[0118] Examples 4 to 11 show that exceeding the optional parameter range will cause the following problems: First, excessive adhesion will cause pore blockage in the diaphragm and poor infiltration of the electrolyte, resulting in a low battery capacity retention rate; Second, too little adhesion will result in poor fit between the diaphragm and the electrode, resulting in a low battery capacity retention rate; Third, the coating will easily stick to the roller during the process, resulting in abnormal battery production.

[0119] The applicant declares that while the process described herein is illustrated by the aforementioned embodiments, this disclosure is not limited to the aforementioned process steps, meaning that implementation of this disclosure does not necessarily rely on these process steps. Persons skilled in the art will appreciate that any improvements to this disclosure, including equivalent substitutions for the raw materials used herein, additions of auxiliary components, and selection of specific methods, fall within the scope of protection and disclosure herein.

Claims

1. An asymmetric composite diaphragm, wherein: The asymmetric composite diaphragm includes a base membrane, a first coating disposed on one side of the base membrane, and a second coating disposed on the other side of the base membrane; The first coating includes a first spherical material grafted with hydrophilic groups; The second coating includes a combination of ceramic particles and a second spherical material.

2. The asymmetric composite diaphragm according to claim 1, wherein: The average particle size of the first spherical material grafted with hydrophilic groups is 100-300 nm.

3. The asymmetric composite diaphragm according to claim 2, wherein: The average particle size of the first spherical material grafted with hydrophilic groups is 200 nm.

4. The asymmetric composite diaphragm according to claim 1, wherein: The monomers grafted with hydrophilic groups include fluorine-containing monomers and / or non-fluorine-containing monomers; The fluorine-containing monomer includes any one or a combination of at least two of polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl chloride, tetrafluoroethylene-ethylene copolymer, polychlorotrifluoroethylene or chlorotrifluoroethylene copolymer; The fluorine-free monomer includes any one of polymethyl methacrylate, polyethyl methacrylate, polymethyl ethyl methacrylate, polybutyl methacrylate, styrene, styrene propene, phenylpropanol, phenylpropanaldehyde or phenylpropionic acid, or a combination of at least two thereof.

5. The asymmetric composite diaphragm according to claim 1, wherein: The grafting rate of the first spherical material grafted with hydrophilic groups is 2-4%.

6. The asymmetric composite diaphragm according to claim 5, wherein: The grafting rate of the first spherical material grafted with hydrophilic groups is 3%.

7. The asymmetric composite diaphragm according to claim 1 or 2, wherein: The coating surface density of the first spherical material grafted with hydrophilic groups in the first coating layer is 0.3-0.7 g / m 2 ; The coating thickness of the first spherical material grafted with hydrophilic groups in the first coating layer is 0.5-1.5 μm.

8. The asymmetric composite diaphragm according to claim 1, wherein: The ceramic particles include any one or a combination of at least two of aluminum oxide, boehmite, magnesium oxide or lithium aluminum titanium phosphate; The average particle size of the second spherical material is 400-600 nm.

9. The asymmetric composite diaphragm according to claim 8, wherein: The average particle size of the second spherical material is 500 nm.

10. The asymmetric composite diaphragm according to claim 1, wherein: The second spherical material is distributed in the second coating layer in the form of islands; The coverage of the second spherical material in the second coating layer is 3-10%.

11. The asymmetric composite diaphragm according to claim 10, wherein: The coverage of the second spherical material in the second coating layer is 5%.

12. The asymmetric composite diaphragm according to claim 1, wherein: The second spherical material includes a fluorine-free material; The fluorine-free material includes any one of styrene acrylic compounds, polymethyl methacrylate, polyethyl methacrylate, polymethyl ethyl methacrylate or polybutyl methacrylate, or a combination of at least two thereof.

13. The asymmetric composite diaphragm according to claim 1, wherein: The coating thickness of the ceramic particles in the second coating layer is 1.5-3 μm.

14. The asymmetric composite diaphragm according to claim 13, wherein: The coating thickness of the ceramic particles in the second coating layer is 3 μm.

15. The asymmetric composite diaphragm according to claim 1, wherein: The coating thickness of the second spherical material in the second coating layer is 2.5-7.5 μm.

16. A method for preparing an asymmetric composite diaphragm according to any one of claims 1 to 15, wherein: The method comprises the following steps: The first spherical material grafted with a hydrophilic group, the first binder and water are mixed once to obtain A first coating slurry is then applied to one side of the base film, and the first coating is obtained after drying once; The second spherical material, ceramic particles, a second binder and water are mixed for a second time to obtain a second coating slurry, and then the second coating slurry is coated on the other side of the base film, and the second coating is obtained after secondary drying.

17. The method according to claim 16, wherein: The grafting method of the first spherical material grafted with hydrophilic groups includes a method of ultraviolet light cross-linking; In terms of weight, the weight of the first spherical material grafted with hydrophilic groups is 5-10 parts, the weight of the first binder is 3-12 parts, and the weight of water is 78-92 parts; The coating method is gravure roller coating; The primary drying temperature is 80° C. and the time is 10 min.

18. The method according to claim 16, wherein: In terms of weight, the total weight of the second spherical material and the ceramic particles is 35 parts, the weight of the second binder is 6 parts, and the weight of water is 59 parts; The mass ratio of the second spherical material to the ceramic particles is (3-12):(88-97); The coating method is gravure roller coating; The secondary drying temperature is 85° C. and the time is 15 minutes.

19. A secondary battery comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein: The diaphragm is an asymmetric composite diaphragm according to any one of claims 1-15.

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